Summary: An important practical question in electrical power engineering is how to determine the peak load of elements of the transmission network. This is so because the peak load determines the parameters of network elements (the cross-section of wires, the rated power of transformers) and eventually – the cost of delivering energy to cus­tomers. This paper presents a theoretical method of determining the peak load, based on the mathematical foundation of adding random variables. It takes into account the number of customers supplied by each element of the network, the forecast peak load for a single customer, and the customer’s mean yearly energy consumption. It critically analyzes and evaluates the current guidelines in this matter, included in the N SEP-E-002 norm, with a particular con­sideration for the values of the coefficients of simultaneous occurrence of peak load for a given number of customers.